Closed-loop control is usually characterized by a two-channel or, respectively, a multi-channel evaluation of the control variable by means of which a rapid and accurate closed-loop control may be implemented. The closed-loop control which is preferably implemented by means of pulse-width modulation comprises a high parasitic signal suppression and at the same time a high band-width.
Many technical processes require a control variable to be maintained on a set value predetermined by a command variable. For this, closed-loop control systems are used in which the control variable is continuously measured and compared with the command variable and, depending on this comparison, adapted by means of correspondingly setting a manipulated variable in terms of an equalization to the command variable. The sequence of actions resulting from this takes place in a closed-loop control system. Depending on the particular application, various physical variables such as pressure, temperature, engine speed, velocity, voltage, current intensity etc. come into consideration. Therein, the components which are characteristic for a closed-loop control system and necessary for the sequence of actions, such as measuring device, comparing device, control device or actuator unit, may turn out to be very different depending on the application. Except for a few exemptions, modern closed-loop control systems are almost exclusively implemented by circuit technique. This is in particular the case for rather complex closed-loop control systems. The spectrum of those closed-loop control systems reaches from basic analog control circuits to digital controllers. Apart from a solution by means of circuit technique, a digital closed-loop control algorithm may also be implemented in the form of a program which runs on a microprocessor or in a field programmable gate array (FPGA). Due to the digital signal processing and the modifiability connected therewith, the digital controller is particularly applicable for rather complex closed-loop control requirements in which a particularly high accuracy and parameters which are reproducible in an accurate manner are essential.
It may be distinguished among other things between continuous controllers and sampling controllers. An analog controller is a typical continuous controller. Since the analog control algorithm may react to changes in the input variable with virtually no time lag and since it may put a corresponding output variable on its output, input and output variables of this type of controller typically consist of continuous signals. In contrast to this, a digital controller is a sampling controller. Its transfer function is implemented by means of a series of arithmetic operations which is carried out successively. Due to the computing time needed in the digital control algorithm, a time lag occurs between measuring the input variable and outputting the output variable. The control variable is not measured continuously but only at certain sampling moments since during the implementation of the control algorithm, the input variable is typically not measured and computed anew. Consequently, digital sampling provides discontinuous time-discrete signals, wherein the signal variable is provided only at discrete points in time. The time between two subsequent sampling moments (cycle time TA) determines the sampling rate or rather the sampling frequency fA. In order to be able to also measure higher-frequent signal parts of the control variable, a high sampling rate is necessary. The upper limit of the sampling rate which is characteristic for a digital controller is predominantly determined by the computing time needed for the computing algorithm. It thus depends on the computing velocity of the used microprocessor, microcontroller or the FPGA, respectively.
For implementing a control system, controllers are required which comprise a control behavior specific to the corresponding application. Therefore, there is a range of simple control elements, the characteristic control properties of which may be described by means of fundamental transfer functions, respectively. By combining several of those control elements, more complex controllers may be constructed, the control behavior of which may be better adapted to the requirements of the corresponding application.
The PI-controller, for example, is a typical controller combination. This type of controller comprises a proportional controller and an integral controller switched in parallel to the proportional controller. Whereas the proportional element multiplies the input value by a fixed factor, the integral element carries out a time integration of the control deviation which is capable of being parameterized. The relatively quick proportional controller therein is a good completion to the integral controller which is in particular responsive to longer-lasting control deviations. Since the PI-controller combines the control properties of its two components, it may react to changes in the control variable or the command variable rather rapidly and it may also lead small static control deviations steady-stately towards zero. A control behavior of this kind is desirable in many technical applications, which is one of the reasons for this controller type being widely spread.
The current control of electric drives is a very important field of application of the PI-controller. Such drives comprise an electric motor as a central component, the electric motor acting as an energy converter, converting the electrical energy supplied to it into mechanical energy. A rotatory motor provides the mechanical energy in form of a rotary motion on a motor shaft, whereas a linear motor provides the mechanical energy as a translation to a movable carriage. Therein, depending on the supplied electrical energy, a certain torque or a certain force, respectively, occurs on the motor shaft or on the motor carriage, respectively, whereupon the motor shaft or the motor carriage, respectively, carries out a motion depending on the effective counter forces. In order to control this motion, the electric drive comprises a current-control device which forms a central closed-loop control system of a drive control. By means of the current-control device which is preferably based on a PI-controller, the current flowing through the motor winding and thus the mechanical energy delivered by the electric motor are directly influenced. In order to portion the electric energy supplied to the electric motor, an actuator unit is used. By means of this actuator unit it is possible to set the forces acting upon the motor shaft or on the motor carriage, respectively, according to the requirement of the PI-controller. Actuator units of modern electric drives use power semiconductors such as power transistors by means of which the supply of electric energy to the motor may be switched on and off.
Position-controlled drives and in particular servo-drives which are used in industrial manufacturing require a very precise current control in order to be able to precisely control the torque or respectively the force and the motion of the servo-motor resulting therefrom. A rapid and precise current control is furthermore required for a high stiffness of the drive and high closed-loop gains of a superimposed rotation-speed control loop. By means of a precise current-control, feed-forward systems may also be efficiently used. Potentially occurring current or torque errors then do not have to be compensated by the slower rotation-speed controller.
The rapid and accurate measuring of the actual value is an exceedingly important property of the closed-loop control system since for a rapid reaction of the closed-loop control system to variations in the control variable the exact knowledge of the effective actual value is necessary. The actual value of the control variable may in principle be measured by different measuring methods, whereby the individual measuring methods partially differ considerably with regard to their accuracy and rapidity. Apart from the continuous measuring of control variables which is in particular characteristic for analog controllers, the control variable may also be measured discontinuously by means of a sampling method. It is common practice in particular for digital controllers to sample the control variable at a sampling frequency which is predetermined by the work cycle of the controller.
In sampling the control variable, however, the sampling theorem has to be taken into account in order to avoid potential measuring errors by higher-frequent parts of the measuring signal. There are several possibilities for this. The control variable may for example be band-limited by means of an anti-aliasing low-pass filter. Herein, high-frequent parts of the measuring signal are filtered out. However, due to the phase shift associated with it, this method is not suitable for all applications. The higher-frequent parts furthermore may be suppressed over a suitable period of time by averaging the measured values. In particular in a control method operating by means of a pulse modulation such as a pulse-width modulation (PWM), a switching period of the pulse modulation may be a suitable period of time. However, the additional downtime in conjunction with the generation of the average value also results in an undesirable phase shift. Eventually, the control variable in a closed-loop control method using a pulse modulation for setting the control variable may also be sampled synchronously with the pulse modulation. This measuring method, however, depends on the existence and on the knowledge of certain harmonic-free points in time of the control variable which renders it very vulnerable to disturbance.